1. Field of the Invention
The present invention relates to an image forming apparatus employing a pre-heating unit to heat a recording medium before the recording medium is transported to a transfer position.
2. Description of the Background Art
Image forming apparatuses such as copiers, printers, facsimile machines, and multi-functional apparatuses including at least two of these functions have been commercially available for some time. Such image forming apparatuses typically use a fixing unit to fix an image onto a recording medium. The fixing unit is either oil-using fixing system or oil-less fixing system. In the oil-using fixing system, a separation film is formed on a fixing roller using silicone oil or the like. In the oil-less fixing system, instead of forming a separation film on the fixing roller, toner-particles including wax are used to enhance separation of toner (e.g., melted toner) from the surface of the fixing roller.
In the oil-less fixing system, a so-called hot-offset phenomenon is more likely to occur during the fixing process due to the physical properties of the waxed toner particles. Hot-offset phenomenon is a phenomenon that toner adheres to a fixing member such as fixing roller although the toner is supposed to be fixed onto a recording medium such as sheet paper. Further, such hot-offset phenomenon is more likely to occur when a fixing temperature of the fixing roller is set higher than usual. Although the higher fixing temperature of the fixing roller is preferred because it enhances glossiness of the output images, the hot-offset phenomenon is more likely to occur at higher temperatures. Accordingly, it becomes hard to increase the fixing temperature of the fixing roller beyond a given temperature level.
In color image forming apparatuses, toner particles having a plurality of colors, superimposed on a recording medium, are melted and mixed to form a given color image. In some cases, the amount of toner adhering to the color image may vary from one portion of the image to another, that is, a toner layer composed of toner particles may vary in thickness, creating some areas of relatively greater thickness. Put another way, a distance between the two surfaces of the toner layer, that is, an upper toner face of the toner layer contacting the fixing roller and a lower toner face of the toner layer contacting the recording medium, is increased. When heat energy is applied to such toner layer in a short time, too great heat energy may be applied to the upper toner face without transmitting sufficient heat energy to the lower toner face, hot-offset phenomenon may occur.
Accordingly, in the color image forming apparatus using the oil-less fixing system, lower heat energy is applied for an extended-time period to an output image to enhance glossiness of the output image. However, such method decreases a transport speed of the recording medium, thus decreasing productivity.
In view of such inconvenience, several approaches have been proposed.
For example, JP-2003-167459-A describes a system using two fixing units. In such configuration, a recording medium carrying an un-fixed toner image is passed through the two fixing units, by which the recording medium carrying the un-fixed toner image can be heated for an extended heating time to enhance glossiness of the output image.
Further, JP-2006-91360-A describes a two-step system to enhance glossiness of the output image. Specifically, a fixing process is conducted for a recording medium carrying an un-fixed toner image, and then a back face of the recording medium is reheated to enhance glossiness of the output image.
Further, JP-2007-187751-A describes a system to heat a recording medium before transferring an image to the recording medium in order to prevent image position displacement between a front face and a back face of the recording medium.
However, in such conventional image forming apparatuses, there is a trade-off among productivity, hot-offset phenomenon, and glossiness. That is, for example, if a higher gloss image is to be produced by increasing the fixing temperature, hot-offset phenomenon may occur; by contrast, when priority is placed on preventing hot-offset phenomenon, productivity may be decreased due to a lower fixing temperature, by which higher gloss image may not be reliably produced. As such, in the conventional image forming apparatuses, enhancement of glossiness of the output image and prevention of hot-offset phenomenon may not be achieved at the same time, or enhancement of glossiness of the output image and higher productivity of image may not be achieved at the same time.
Specifically, in JP-2003-167459-A and JP-2006-91360 discussed above, hot-offset phenomenon is more likely to occur when heat energy is applied to an upper face of the toner image formed on the recording medium during a second fixing process. Further, the toner particles of the toner image on the recording medium may become gel when the recording medium is transported from a first fixing unit to a second fixing unit, wherein the first fixing unit is set at an upstream of a transport route of the recording medium, and the second fixing unit is set at a downstream of a transport route of the recording medium. When the toner particles of the toner image on the recording medium become gel, the toner is more likely to adhere to parts configuring the transport route, which is undesirable.
In JP-2007-187751-A, the recording medium is heated before transfer of an image to the recording medium, thus enhancing glossiness of the output image for the purpose of preventing image position displacement between a front face and a back face of the recording medium. However, the temperature to which the recording medium is heated is uncontrolled for image forming process, and therefore does not provide a complete solution to the above-described problems.